In many chemical processes, it is important to be able to detect even very low levels of impurities in the feed streams to the process so that, if necessary, appropriate steps can be taken to remove, and/or mitigate the deleterious effect of, these impurities. For example, it is well known that zeolite catalysts are highly sensitive to basic impurities, such as nitrogen compounds and arsine, particularly when used in low temperature, liquid-phase processes, such as the liquid-phase alkylation of benzene with C2 to C4 olefins.
Various methods are currently available for detecting impurities in feed streams, but in general these methods have limited utility with low concentrations of impurity and/or have limited effectiveness with certain important combinations of feed stream and impurity type. For example, chemiluminescence detection is a standard method of detecting a wide range of nitrogen impurities in liquid hydrocarbons, but its detection limit is normally of the order of 0.3 ppm of nitrogen. Moreover, chemiluminescence is not a reliable method of detecting nitrogen impurities in light olefinic feedstreams due to their relatively high heat of combustion which may cause interference in the detection system. Similarly, although ion selective electrodes can be used to detect low levels of specific nitrogen impurities, such as ammonia or hydrogen cyanide, in olefinic feed streams, the method does not work well with light alkyl amine impurities. In addition, both chemiluminescence and ion selective electrodes are part of complex procedures requiring expensive laboratory instrumentation and facilities, and so cannot readily be employed in situ at chemical process sites.
In fact, there is currently no generally accepted standard method available for the detection of nitrogen impurities in light hydrocarbon gases below 0.1 ppm. There is therefore a need for an improved method of detecting and measuring the level of impurities, such as ammonia, alkylamines, nitriles, cyanides and arsine, in gas streams, such as light hydrocarbon (C1 to C4) streams, especially when the impurities are present at very low levels, for example less than 0.5 ppm and even less than 0.05 ppm.
In the field of air quality monitoring, it is known to test for a given component by drawing a small specified amount of air, typically 100 to 400 cc, through a gas detection tube with a hand-operated pump. The tube contains a reagent which reacts selectively with the component to be measured to produce a compound which either directly or indirectly produces a color change. By measuring the extent of the color change, the amount of the component in the air sample can be determined. Such gas detection tubes are commercially available from, for example, Gastec Co. and Kitagawa Co.
According to the present invention, a method of measuring the level of an impurity in a gas stream is provided which employs a gas detection tube of the type normally used in air quality monitoring but which allows a large, measured quantity (typically 1 to 20 liters) of gas to pass through the detection tube, instead of the 100 to 400 ml employed in air quality monitoring. In this way, the level of the impurity in the gas stream can be determined in a simple and inexpensive manner well-suited to in situ application at a chemical process site while at the same time the minimum detection limit of the tube can be lowered significantly.